Multilayer papery materials are known, and in particular so-called “corrugated cardboard”, which comprises a sheet of corrugated paper, i.e. shaped to present a continuous succession of waves, and a flat paper sheet glued to the corrugated paper sheet along its crests. Two flat paper sheets can also be glued, one onto each side.
Whether it presents one, two or more flat sheets, the known corrugated cardboard is mainly used in the technical packaging sector, in which the function of the corrugated paper layer is to protect the packaged products, while the function of the flat sheet or sheets is to stiffen the corrugated paper layer and to form a support for the printing.
In known methods for producing the corrugated paper layer the wave crests are always transverse to the machine direction, i.e. perpendicular to the advancement direction of the paper web forming the corrugated layer. Because of this, the known corrugated cardboard has greater rigidity to bending in the transverse direction and lesser rigidity to bending in the longitudinal direction.
In addition to being used for wrapping the most varied products, the known corrugated cardboard is also used to form boxes and containers in general, to hold the articles to be packaged, these not necessarily being the same shape as the container, hence their poor bending rigidity in the longitudinal direction gives rise to the difficulty of producing sufficiently rigid containers.
To reduce this problem, it has previously been proposed to increase the number of corrugated paper layers, hence substantially increasing the weight and cost of the packaging.